This invention generally relates to Fischer-Tropsch synthesis, and more specifically, to a safe method for both transporting Fischer-Tropsch syncrude from a remote production site to a developed site and supplying salable products from the developed site to the remote site.
Crude oil is a mixture of hydrocarbons when it comes out of the ground. Typically, the mixture is separated into at least three fractions, a gas fraction, an intermediate fraction, and a crude oil fraction, which tend to have some degree of overlap. The gas fraction includes mostly C1-3 hydrocarbons, the intermediate fraction includes mostly C3-5 hydrocarbons, and the crude oil fraction includes C4+ hydrocarbons.
Crude oil is often obtained at locations far from where it is ultimately converted into distillate fuel compositions and other products. Crude oil is transported to commercial refineries after the gas and intermediate fractions, and optionally naphtha fractions, have been removed. Crude oil fractions must have a relatively low vapor pressure when they are transported because of safety regulations as well as practical limits on the pumping and storage of volatile crude oil. However, it is common practice to ship crude oil that includes volatile components in concentrations that do not cause the crude oil to exceed the vapor pressure specification or increase the API (American Petroleum Institute) gravity excessively. Petroleum-based crude oil, therefore, typically includes C5+ hydrocarbons, with an amount of butane that will not cause the crude oil to exceed the vapor pressure specification. Propane and lighter hydrocarbons are avoided because of their volatility.
The volatility of crude oil in commercial tankers is typically limited to about 9 psia (pounds per square inch absolute) when measured at the shipping temperature. International maritime regulations limit the maximum Reid Vapor Pressure of crude oil carried aboard conventional tankers to xe2x80x9cbelow atmospheric pressurexe2x80x9d (i.e., less than 14.7 psia). These same regulations limit the closed cup flash point xe2x80x9cnot to exceed 60xc2x0 C.xe2x80x9d (Safety of Life at Sea (SOLAS), Chapter 22, Regulation 55.1). A practical operational limit is a True Vapor Pressure, not Reid Vapor Pressure, of about 9-10 psia for conventional tankers. A True Vapor Pressure higher than approximately 10 or 11 psia during pumping will make it difficult, if not impossible, to fully discharge the tanker""s cargo tanks, although the actual pumping performance will depend on the particular ship. Receiving shoreside terminals commonly have a maximum True Vapor Pressure limit of 11 psia, based on the maximum capability of floating roof storage tanks.
Waxy crude oils typically do not contain significant quantities of volatile components and can be shipped at temperatures up to around 160xc2x0 F. without exceeding the maximum vapor pressure. Slack waxes from petroleum deoiling and dewaxing operations can also be shipped by tanker in a molten state. These waxes include mostly high molecular weight hydrocarbons and do not typically include significant amounts of volatile light components that would cause problems with excessive vapor pressure when the waxes are molten. Accordingly, a preferred method for transporting such waxes is in the molten state.
Like crude oil, natural gas is often obtained at locations far from where it is ultimately converted. It is often more commercially feasible to convert the natural gas into higher molecular weight hydrocarbons at remote locations than to transport the natural gas to another location for conversion. Many processes, such as Fischer-Tropsch synthesis, can be used to convert methane into higher molecular weight hydrocarbons. Fischer-Tropsch synthesis involves the initial conversion of methane into synthesis gas or xe2x80x9csyngas,xe2x80x9d and the subsequent conversion of syngas into higher molecular weight products. Because it is desirable to limit the amount of processing equipment at remote locations, the Fischer-Tropsch products may be hydroprocessed at commercial refineries far from where the Fischer-Tropsch synthesis is performed.
The products of Fischer-Tropsch synthesis are mostly linear hydrocarbons that often include high melting point wax. A C5+ product stream, commonly referred to as xe2x80x9csyncrude,xe2x80x9d can be isolated. At the Mossgas facility in South Africa and the Shell facility in Malaysia, both developed sites with low to moderate capital costs, methane is converted into syncrude, which is refined at the site into finished salable products. When capital costs at remote sites are high, the syncrude could also be transported to commercial refineries for hydroprocessing, for example by hydrocracking, hydroisomerization and hydrotreatment, to produce products with desired properties. This minimizes the construction of expensive facilities at remote sites.
Methods for transporting Fischer-Tropsch derived syncrude from a remote site to a commercial refinery are known in the art. One approach has been to isolate a C20-36 syncrude and ship this composition as a solid. A limitation of this approach is that it is difficult and expensive to transport solids, because such transportation requires expensive forming, loading and unloading facilities.
Another approach involves transporting the syncrude as a molten wax. This transportation method does not require the forming, loading and unloading facilities needed to transport solids or the dewaxing facilities needed to convert the syncrude into a product that is liquid at room temperature. However, Fischer-Tropsch products include a sufficient quantity of volatile hydrocarbons that would cause the products to exceed the vapor pressure specifications if the syncrude were shipped at a temperature at which the syncrude is molten.
Other approaches have focused on transporting syncrude, or syncrude that has been partially refined to convert some of the linear hydrocarbons into isoparaffins and thus generate syncrude that is liquid, at near ambient temperature. One approach to transporting syncrude in the liquid state involves partially dewaxing the syncrude to form a pumpable liquid (see, for example, U.S. Pat. No. 5,292,989). However, this dewaxing may require the construction of facilities that are expensive and difficult to operate in remote locations.
There exists demand for salable products, such as gasoline, distillate fuels, solvents, lubricants, etc., both at the remote sites where natural gas is converted into syncrude and in their surrounding communities. Fischer-Tropsch syncrude will be waxy and will also contain volatile components, complicating the shipping of both Fischer-Tropsch products from remote production sites to developed sites and salable products from developed sites to remote sites.
It would be advantageous to provide a safe and efficient method for manufacturing and transporting Fischer-Tropsch syncrude in the liquid state without requiring dexwaxing conditions and without exceeding the vapor pressure specifications in the transportation method from remote production sites to developed sites and supplying salable products from developed sites to remote sites. The present invention provides such a process.
A novel transportable Fischer-Tropsch liquid syncrude and a safe and efficient method to manufacture and transport the product from a remote production site to a developed site and to supply salable products from the developed site to the remote site are disclosed.
The novel Fischer-Tropsch liquid syncrude, which may be produced at a remote site, differs from conventional Fischer-Tropsch liquid syncrude that contains both volatile and waxy components wherein the mixture has a true vapor pressure in excess of about 15 psia when measured at the transportation temperature.
The method of the invention involves converting a light hydrocarbon feedstock into syngas, converting the syngas into syncrude via Fischer-Tropsch synthesis, and separating the Fischer-Tropsch liquid syncrude into at least one waxy fraction, with a pour point in excess of 20xc2x0 C., and at least one non-waxy fraction. The fractions have true vapor pressures of less than about 15 psia, preferably less than 11 psia, when measured at their transportation temperature and contain greater than 60%, preferably greater than 75%, linear hydrocarbons by weight. At least two of the fractions, including at least one non-waxy fraction, are supplied to a vessel in substantially liquid form, wherein they are separately transported to a developed site where they are unloaded.
A C1-2 fraction can also be isolated from the Fischer-Tropsch synthesis and recycled upstream of syngas generation, flared, used to produce hydrogen, and/or used for fuel. A C3-enriched fraction, including more than 5% by weight C3, preferably more than 20% by weight C3, and most preferably more than 40% by weight C3, can also be obtained. This fraction can be recycled upstream of syngas generation, flared, used for fuel, transported in pressurized tankers, and/or transported in refrigerated tankers.
In a preferred process and business method embodiment, petroleum products such as crude oil can be blended with one or more of the transportable Fischer-Tropsch products, so long as the resulting blended composition has a true vapor pressure of less than about 15 psia, preferably less than 11 psia, when measured at its transportation. The resulting composition preferably has a pour point in excess of 20xc2x0 C. The blending of fractions can occur at any site: the remote Fischer-Tropsch site, a lube manufacturing site, a distillate refinery site, or another location.
The products can be transported using any commonly used means of transportation, including marine tankers, rail cars, pipelines, trucks, barges and combinations thereof. A preferred means of transportation is a marine tanker. Whether transported by marine tanker, rail, truck, and/or barge, each transportable product is transported either in a separate marine tanker, rail, truck, or barge, or in a separate zone, isolated by internal partitions, for each transportable product within the same marine tanker, rail, truck, or barge.
After unloading the separate fractions at the developed site, at least one vessel or portion of a vessel that contained a non-waxy component is filled with at least one salable product. The vessel is then returned to the remote site, and the salable product is unloaded.